Infective endocarditis in dogs in the UK: 77 cases (2009‐2019)

Objectives To determine the causative organisms, clinical features and outcome of canine infective endocarditis in the UK. Materials and Methods Medical records of three veterinary referral hospitals were searched for dogs with infective endocarditis between December 2009 and December 2019. Signalment, clinical signs, causative organism, valve affected, treatment and survival data were recorded. Results Seventy‐seven cases with possible or definite infective endocarditis (according to the modified Duke criteria) were included. The majority were large breed (40/77 – 51.9%). There were 47 of 77 (61%) male dogs and the mean age was 7.3 ±3 years. A causative organism was identified in 26 of 77 (33.8%) cases. The most common organisms were Escherichia coli (7/27 – 25.9%), Pasteurella spp. (5/27 – 18.5%), Staphylococcus spp. (4/27 – 14.8%) and Corynebacterium spp. (4/27 – 14.8%). Bartonella spp. were not detected in any patients. The mitral valve was most commonly affected (48/77 – 62.3%). Clinical features were non‐specific, with lethargy being the most common clinical sign observed (53/77 – 68.8%). Fifty‐three dogs (68.8%) survived to discharge. The median survival time post discharge was 425 days (2 to 3650 days). The development of congestive heart failure was associated with a poorer outcome. Cardiac troponin concentration, antithrombotic use and the development of thromboembolism or arrhythmias were not significantly associated with outcome. Clinical Significance Some dogs with infective endocarditis that survive to discharge can have a long lifespan. The inability to detect an underlying organism is common and Bartonella spp. may be a less prevalent cause of canine infective endocarditis in the UK than in the USA.


INTRODUCTION
Infective endocarditis (IE) is a life-threatening disease that is difficult to diagnose and manage in veterinary patients (Miller et al. 2004). It is caused by bacterial infection of the valvular endothelium and results in proliferative or erosive lesions leading to valvular insufficiency (Häggström 2010). The prevalence of IE varies between publications but is considered low in canine patients. An incidence of <1% has been reported in one veterinary hospital (MacDonald et al. 2004). Males appear to be at a IE can be difficult to diagnose ante-mortem due to the nonspecific and variable clinical signs and limited diagnostic capabilities in general practice (Häggström 2010). However, a modified version of the human Duke criteria used for diagnosis of IE in dogs has been described (Sykes et al. 2006a). Although not part of the Duke criteria, cardiac troponin-I (cTnI) is another supportive test for IE (Kilkenny et al. 2021). The most common bacteria identified in previous studies of dogs with IE were Streptococcus spp. and Bartonella spp. according to two case series in the USA (Sykes et al. 2006a, Reagan et al. 2022. Other commonly implicated organisms include Staphylococcus spp., Escherichia coli, Pseudomonas spp., Erysipelothrix rhusiopathiae, Pasteurella spp. and Corynebacterium spp. (Peddle & Sleeper 2007, Reagan et al. 2022. Previous studies indicate that Streptococcus spp. most commonly infects the mitral valve, Bartonella spp. tend to affect the aortic valve while Staphylococcus spp. display no valve predilection (MacDonald et al. 2004, Sykes et al. 2006a. Other Gram-negative bacteria showed a predilection to infect the mitral valve (Sykes et al. 2006a). A bacteraemia is required for the development of IE; however, many cases have no clinically detectable source of infection, possibly because many dogs are already receiving antibiotic therapy before the start of a diagnostic work-up (Romero-Fernandez & Palermo 2019). The use of antithrombotics has been shown to increase survival time in IE patients (Reagan et al. 2022).
The prognosis for IE and its sequelae in dogs is guarded; in one retrospective case series, a survival rate of 50% was reported (Reagan et al. 2022). This appears to be dependent on the valve affected, however, with a shorter median survival time of just 3 days in dogs with aortic valve IE and of 476 days in dogs with mitral IE according to a previous study (MacDonald et al. 2004). The shorter survival time of aortic valve infections is thought to be due to its predisposition to Bartonella spp. colonisation (Sykes et al. 2006b). This can lead to aortic regurgitation which is less well tolerated than mitral regurgitation as it is associated with high afterload and possibly myocardial failure. A recent study has found a longer survival time of 71 days in dogs with aortic valve IE due to Bartonella spp. than previous studies (Reagan et al. 2022). Complications associated with IE include congestive heart failure (CHF), immune-complex disease and thromboembolic disease (TED) which can manifest in many organs including the kidneys (Reagan et al. 2022). The development of CHF, TED and acute kidney injury (AKI) has been shown to be negatively correlated with survival (Reagan et al. 2022).
The scientific literature on canine IE is limited and focused on veterinary hospitals in the USA. The aim of this study was to address the gap in the literature on canine IE cases in the UK, specifically to describe signalment, presenting clinical signs, valve affected, causative bacterial species and outcome in these patients.

Study design and inclusion criteria
This was a retrospective study and the medical records of three UK veterinary referral hospitals were searched for dogs diag-nosed with IE between January 2009 and December 2019. Cases were classified as definite or possible IE based on the modified Duke criteria described by Keene (2002), Sykes et al. (2006a) and Ljungvall & Häggström (2017) (Table 1) or definite when the diagnosis was confirmed by post-mortem examination. Cases were excluded if they had had previous cardiac surgery associated with the mitral valve repair programmes at two of the veterinary hospitals. Positive findings for IE on echocardiogram as described in Table 1 involved documenting changes in the normal heart anatomy such as thickening of the valves, vegetative lesions, which are often irregularly outlined and oscillatory (i.e. move independently from the valve) and associated valvular insufficiencies and/or elevated valve velocities (Ljungvall & Häggström 2017). The mitral valves were viewed from several angles to help distinguish between myxomatous nodular lesions (if degenerative valvular disease was present) and vegetations (Ljungvall & Häggström 2017). The echocardiograms were carried out by residents or board-certified veterinary cardiologists at the veterinary hospitals. A simultaneously acquired single-lead electrocardiogram (ECG) was reviewed during echocardiography, with a six or 12 lead ECG recorded according to clinical indication or clinician's preference. The presence (or absence) of arrhythmias was noted.

Medical record search
Electronic medical records from each referral hospitals were searched for dogs diagnosed with endocarditis between 2009 and 2019. The software used included VetCompass, Tristan and Rx-Works. Medical records were searched using the keywords "endocarditis" in three centres and "new heart murmur," "pyrexia" and "lethargy" in one centre. The medical records were searched by two operators in one centre and one operator each in both other centres. The dates the medical records were searched were May and July 2021.

Data extracted from records
Clinical features were recorded including patient signalment, presenting clinical signs, Duke criteria fulfilment, microbial culture of blood (MCB) and Bartonella spp. polymerase chain reaction (PCR), valves involved, circulating cTnI concentrations, antibiotic therapy (before and post IE diagnosis), other therapy started post IE diagnosis, any comorbidities, hospitalisation length, complications (development of CHF, TED, arrhythmias and renal complications) and patient outcome. To evaluate outcome for dogs that survived hospital discharge, the primary veterinary practices of patients were contacted to determine whether they were known to be alive, or if they had died or were euthanased. The date of their euthanasia or natural death was gathered to the nearest month. If a dog died and a post-mortem consent was provided, post-mortem examination was carried out. Bacterial culture samples were taken aseptically from cardiac tissue. Ethical approval was gained to contact the veterinary practices from all institutions (ethical review reference number SR2020-0120; VREC1092). The ethical approval SR2020-0120 covers two centres. The development of complications was recorded as follows; CHF was diagnosed either on post-mortem by board-certified clinical pathologists or by findings of cardiogenic pulmonary oedema (e.g. enlarged cardiac silhouette, enlarged pulmonary vasculature and infiltrative pulmonary patterns) by thoracic radiographs identified by board-certified veterinary radiologists. Thromboembolic events were defined as visualisation of infarcts or thrombus either at post-mortem examination or by abdominal ultrasound or computed-tomography by board-certified clinical pathologists or radiologists respectively. Renal complications were defined as cases with serum creatinine concentrations above the normal reference range in animals with concurrent isosthenuria or hyposthenuria and no history of chronic kidney disease.

Bartonella detection
Detection of Bartonella spp. for this study was carried out by DNA extraction and qPCR for centre A, PCR alone for centres B and C.

Collection of blood cultures
In centre A, aseptic collection of three, 3 to 10 ml aliquots were collected from three different veins following sterile preparation, all taken at the same time. The whole blood was then subcultured onto blood agar and MacConkey agar for aerobic and anaerobic cultures and incubated at 37°C for 7 days. In centre B, aseptic collection of three, 5 ml aliquots were collected from three different veins following sterile preparation in a time frame of 60 minutes. The whole blood was then subcultured onto blood agar for aerobic and anaerobic cultures and incubated at 37°C for 7 days. In centre C, aseptic collection of three, 2 to 5 ml aliquots were collected from three different veins following sterile preparation, 30 minutes apart. The whole blood was then subcultured onto Signal Blood Culture System (ThermoFisher) for aerobic and anaerobic cultures and incubated at 38°C for 4 to 7 days.

Statistical analysis
Continuous variables were assessed for normality using the Shapiro-Wilk test. Normally distributed data were reported as mean ±sd and non-normally distributed data as median (minimum to maximum range). Data were analysed using the statistical analysis programme GraphPad Prism Version 9.0 (GraphPad Software). Definite and possible cases of endocarditis were analysed together. Kaplan-Meier survival curves were constructed and the log-rank test was used to compare the following populations: dogs with different valve infections, use of antithrombotics, development of CHF, TED and arrhythmias. Survivors were censored on the last day of follow-up. Cases lost to follow-up before 1 month after discharge were excluded from the patient outcome analysis. Values of P < 0.05 were set as significant.

RESULTS
The medical record search identified 287 patients at referral centre A, 49 records at centre B and 42 records at centre C that were eligible for assessment. In total, 77 cases were eligible for the study, the rest were excluded as the final diagnosis was not IE and they did not fulfil enough criteria to be defined as possible or definite cases of IE.

Common clinical signs
The median duration of illness before admission was 7 days (0 to 334 days). The most common clinical signs on admission are summarised in Table 2. Other clinical signs recorded were blindness, ptyalism and epistaxis in one dog each. Seventy-one (92%) dogs presented with multiple clinical signs.

Duke criteria fulfilment
Out of 77, 67 (87%) dogs were classified as definite endocarditis, and 10 (13%) as possible according to the modified Duke criteria. Exclusion of possible cases did not impact the results or statistical analyses. Figure 1 summarises how many cases fulfilled each of the modified Duke's criteria. Three dogs had prolonged intravenous (iv) catheterisation sites and one had an infected iv catheter site. One of the dogs with the prolonged catheterisation sites had a vascular access port placed. This dog subsequently developed a tricuspid IE. The dog with the infected catheter site developed aortic IE and the other two dogs developed mitral IE.
Ten dogs were submitted for post-mortem at which point a definite diagnosis of endocarditis was confirmed by bacterial culture of cardiac tissue and characteristic valve pathology. Microbiology laboratory reports of these samples did not interpret any of the cultures to be potentially contaminated.
Eight of the 10 dogs were already diagnosed as definite endocarditis cases before post-mortem examination. However, the remaining two cases were initially classed as "possible" endocarditis cases before the post-mortem examination. One of these dogs did not have an MCB submitted ante-mortem and another had a negative MCB result. Following confirmation of the post-mortem results, these two dogs were then classified as "definite" endocarditis. In four cases, the records did not state how many MCBs were collected; these were classed as a single positive MCB.

Infecting organism and valve involvement
A causative organism was identified in 26 of the 77 cases (34%). Seven dogs had multiple organisms grown on MCB (in four cases this was detected on post-mortem examination). Figure 2 shows the distribution of organisms confirmed by blood culture and which valve they infected. There was a negative blood culture or blood cultures in 36 (59%) of 77 dogs, with this being more common in referral centre B (n=7/8, 87.5%) compared to referral centre A (n=13/32, 37.5%) and referral centre C (n=16/22, 72.7%). One dog had no blood culture taken but IE was confirmed by post-mortem examination. No blood culture or postmortem examination was performed in 14 (18.2%) of the 77 cases. Six of these cases died or were euthanased within 3 days of admission (range 1 to 5 days). Two of these 14 dogs were classified as possible endocarditis, the other 12 were classified as definite endocarditis. Table 3 shows how the 12 cases that were classified as definite endocarditis fulfilled this classification despite not having a blood culture or post-mortem carried out. A PCR test for Bartonella spp. was performed for 13 dogs, the results were negative for all 13.   Out of the 77 cases, the mitral valve was infected in 48 cases (62.3%), the aortic in 18 cases (23.7%) and the tricuspid in two cases (2.6%). The aortic and mitral were both infected in six cases (7.9%), while the aortic and tricuspid, and the aortic, tricuspid and pulmonic were infected in one case each (1.3%). One dog did not have an echocardiogram done. This dog was classified as possible endocarditis as it did not fulfil any of the major criteria and had four minor criteria. Six dogs had mural lesions in addition to a valve lesion (8%): two dogs with aortic IE had a lesion on the right interatrial septum, one dog with aortic IE had a lesion extending into the right atrium, one dog with aortic and tricuspid IE had a lesion extending into the myocardium of the atrioventricular region, one dog with mitral IE had an lesion extending into the myocardium of the left ventricle and one dog with mitral IE had a lesion in the ventricular apical lumen and another lesion extending into the left ventricular outflow tract.
Hospitalisation and patient outcome Out of the 77 patients, 19 dogs were euthanased and five died spontaneously at the hospital (30%). Of the 19 that were euthanased, 13 (68%) had mitral IE, four (21%) had aortic IE, one (5%) had tricuspid IE and one had mitral and aortic valve IE (5%). Of the five that died, three (60%) had mitral IE and two (40%) had aortic IE. The median hospitalisation length of cases that died or were euthanased was 2 days (0 to 10 days). Of the 77 cases, 53 (69%) survived to discharge with a mean length of hospitalisation of 7.1 days ±3.9 days. Dogs that were discharged from the hospitals with aortic valve endocarditis lived a median of 480 days (range 22 to 3650 days) while dogs with mitral valve endocarditis lived a median of 440 days (range 2 to 2769 days) (Fig 3); survival times were not significantly different for the site of endocarditis. Fifteen dogs (28%) were lost to follow-up and excluded from this analysis. The dog with tricuspid valve endocarditis lived 152 days. Dogs with both aortic and mitral valve endocarditis lived a median of 121 days (2 to 1065 days) and the dog with aortic, tricuspid and pulmonic valve endocarditis lived for 1825 days.
The median hospitalisation time of dogs with mitral valve IE was 5 days (n=48, 0 to 15 days), for aortic valve IE it was 4.5 days (n=18, 1 to 11 days), and for tricuspid valve IE it was 4.5 days (n=2, 0 to 9 days). The mean hospitalisation length of dogs with mitral and aortic valve IE was 7 days (n=5, ±4.5 days). The hospitalisation time of the dog with aortic and tricuspid valve IE was 8 days and the dog with aortic, tricuspid and pulmonic valve IE was 5 days. The mean hospitalisation time of dogs with single valve IE was 5.7 days (n=68, ±3.8 days) and that of multiple valve IE was 6.9 days (n=7, ±3.8 days).

Antimicrobial therapy
Of the 77 cases, 52 (68%) had received either injectable or oral antimicrobial therapy when they presented at the referral hospitals, as summarised in Table 4. The median time antimicrobial therapy was prescribed by the referring veterinary practice was 7 days before referral to the hospitals (1 to 56 days). Topical antibiotics were excluded from this analysis.
Of the 52 dogs that received antimicrobial therapy before referral, 24 subsequently showed negative MCB at the referral hospitals (46%). Of the remaining 25 dogs that had not received antibiotics before referral, 12 had negative MCB (48%).
The most common antibiotic therapies prescribed at the referral centres were amoxicillin-clavulanic acid (Synulox;  and metronidazole (n=4, 6%). These were administered either iv, subcutaneously or by mouth. Nine (12%) of the 77 dogs did not receive antibiotic treatment at the hospital as they either died or were euthanased before therapy was started.
Of the 53 dogs that survived to discharge, the most common antibiotic protocol prescribed once discharged was 2 to 12 weeks of amoxicillin-clavulanic acid and a fluoroquinolone (n=23, 43%) by mouth. Four dogs had markedly prolonged therapy for periods of 5 to 22 months. Amoxicillin-clavulanic acid, enrofloxacin and metronidazole was used in nine dogs (17%). Table 5. Other therapies used included metoclopramide (Emeprid; CEVA), mexiletine (Mexiletine HCl; Summit) and amiodarone (Amiodrone; Covetrus) in one case each. Therapies intended solely for analgesia were excluded from this analysis, e.g. non-steroidal anti-inflammatory drugs, opioids etc. Antithrombotic medication (clopidogrel and/or aspirin) was used in 18 (23%) of 77 dogs, four of which developed TED. Figure 4 summarises the survival curves between dogs that received antithrombotic medication and those that did not. Sixteen dogs were excluded from this graph as they were lost to follow-up. The log-rank test showed no significant difference in the survival time between these groups.

Complications of IE
Eleven (14%) out of 77 dogs developed CHF. These included nine (82%) dogs that developed left-sided CHF, five (56%) of which were mitral valve IE, two (22%) had mitral and aortic valve IE and two (22%) had aortic valve IE. One (9%) dog devel-oped biventricular CHF with an aortic valve IE and one (9%) dog developed mitral valve IE but the post-mortem analysis did not specify what side CHF the dog developed. The median survival time of dogs that developed CHF with IE was 5 days (range 0 to 908 days). Figure 5 summarises the survival curves between dogs that developed CHF and those that did not. Fifteen dogs were excluded from this analysis as they were lost to follow-up. The log-rank test showed a significant difference between the survival time of these two groups (P=0.0440).
Sixteen (21%) out of 77 dogs developed TED. Eight (50%) dogs had renal TED, seven (43%) dogs had splenic TED, four (25%) dogs had TED in their musculature, two (13%) dogs had liver TED and one (6%) dog had an aortic TED. Some dogs developed TED in multiple locations. Figure 6 summarises the survival curves of dogs that developed TED and those that did not. Fifteen dogs were excluded from this analysis as they were lost to follow-up. The log-rank test showed no significant difference between the survival time of dogs that developed TED and those that did not.
Twenty-seven (35%) out of 77 dogs developed arrhythmias. Ventricular arrhythmias recorded included ventricular prema-   ture complexes (n=14, 52%), accelerated idioventricular rhythm (n=12, 44%), ventricular tachycardia (n=4, 15%) and ventricular bigeminy or trigeminy (n=2, 7%). Atrial arrhythmias recorded included supraventricular tachycardia (n=4, 15%) and supraventricular premature complexes (n=2, 7%). Four dogs had atrioventricular block, which was characterised as first degree in three dogs, second degree in one dog and third degree in one dog. Some dogs showed multiple types of arrhythmias. Figure 7 summarises the survival curves of dogs that developed arrhythmias and those that did not. Fifteen dogs were excluded from this analysis as they were lost to follow-up. The log-rank test showed no significant difference between the survival times of dogs that developed arrhythmias and those that did not. AKI was observed in four (5%) of the 77 cases and the median survival time of these cases was 2 days (range 0 to 908).
One dog was diagnosed with a tract connecting the left ventricle and right atrium (Gerbode defect), presumed as a complication of IE. This was diagnosed on echocardiographic examination and confirmed at post-mortem. This dog had aortic valve IE and had been diagnosed with congenital SAS. The dog subsequently developed third degree AV block and was euthanased on the second day of hospitalisation due to clinical worsening. Table 6 summarises the cTnI concentrations in the 30 dogs in which measurements were taken.

DISCUSSION
This multi-centre study represents the first review of canine IE in a referral population of dogs in the UK and the second largest case series of IE to date. Large and medium breed dogs appear to be more predisposed to developing IE than small breed dogs, as has been described in previous veterinary studies (Sisson & Thomas 1984, Peddle & Sleeper 2007, Kilkenny et al. 2021, Reagan et al. 2022. However, reasons for this remain unclear. The mean age at which dogs were infected with IE in this study was similar in males and in females. A higher proportion of middle-aged to older dogs were reported with IE in this study as noted in previous studies (Sisson & Thomas 1984, Sykes et al. 2006b, Kilkenny et al. 2021, Reagan et al. 2022). This may be due to age-related senescence of the immune system, which has been shown to increase the incidence of infection in older pets (Day 2010). Similar to previous studies, we show that male dogs have a greater predisposition to developing IE than female dogs (Sisson & Thomas 1984, MacDonald et al. 2004, Reagan et al. 2022. Studies have shown sex differences in immune components with female dogs displaying stronger cell-mediated and humoral responses, greater numbers of CD8 T-cells and higher immunoglobulin levels than males which may account for this difference (Blount et al. 2005, Sundburg et al. 2016. The most common organisms that were cultured in this study were E. coli, Staphylococcus spp. and Pasteurella spp., which have all been reported in previous USA studies ( MacDonald et al. 2004, Sykes et al. 2006a, Reagan et al. 2022. The mitral valve was most commonly infected in this study as shown in recent studies (Kilkenny et al. 2021;Reagan et al. 2022). This differs from previous studies however where both the aortic and mitral valve were frequently affected (Sykes et al. 2006a). One of the reasons for this is likely linked to the lack of Bartonella spp. IE cases, which appear to preferentially affect the aortic valve (MacDonald et al. 2004). However, it is also possible that Bartonella infections were missed due to a low level of PCR testing (in only 17% of cases), particularly given the high level of cases where no causative organism was detected (66%). A recent study found a 3% seroprevalence of Bartonella spp. in UK dogs (Alvarez-Fernandez et al. 2018). A similar seroprevalence was found in USA dogs at 3.6%; however, this increased to 36 and 52% when dogs were coexposed to Ehrlichia canis or Babesia canis, respectively (Alvarez-Fernandez et al. 2018). Neither E. canis or B. canis are thought to be endemic in the UK, which may explain why Bartonella spp. were not detected in our patients (Bird 2016, Wright 2018. Research shows that PCR testing is no more sensitive at detecting Bartonella spp. than blood cultures (Meurs et al. 2011, Roura et al. 2018, however, this depends on what samples were used to run the PCRs and how the blood samples were cultured. Studies have shown that using only valve tissue samples rather than blood samples and a pre-enrichment culture before PCR testing may increase Bartonella positive results (MacDonald et al. 2004, Davis et al. 2020. A recent study utilised serology, PCR and blood cultures to aid their identification of Bartonella spp. as a cause of IE (Reagan et al. 2022). These techniques were not utilised in this study. Thus, performing both MCB, serology and PCR simultaneously may improve the detection of Bartonella spp. in IE patients, and may be required to prove that Bartonella spp. is not a major cause of IE in the UK (Meurs et al. 2011).
Nearly, half the MCBs in this study were negative and this was not related to antimicrobial therapy before referral. This was shown by the lack of differences in the number of MCBs between groups that did and did not receive antimicrobials before referral. This may be due to the ability of some bacteria to invade macrophages and reside in cells as quiescent intracellular reservoirs, which may help protect it against the immune system and antimicrobial therapy (Croxen & Finlay 2009). Other common reasons for obtaining negative MCBs include infections by nonbacterial organisms such as Aspergillus spp. or fastidious organisms such as Chlamydia spp. or Mycoplasma spp. which have been shown to cause endocarditis in humans (Sykes et al. 2006a, Habib et al. 2010. Aspergillus spp. were cultured in two dogs in this study; unfortunately, its diagnosis can be missed as it is a slow-growing organism and therefore takes longer to isolate from MCBs (Pasha et al. 2016). Fungal endocarditis lesions have been shown to embolize easily in humans and should therefore be suspected in patients with negative MCBs and signs of embolic disease.
Successful treatment of IE is based on early diagnosis and immediate, aggressive treatment to minimise secondary complications. Selection of the appropriate treatment is based on culture and sensitivity testing; however, while the culture results are pending, empirical treatment with a broad-spectrum antibiotic such as an aminoglycoside, beta-lactam or fluoroquinolone is recommended (Häggström 2010). The most common antibiotic therapy protocol used in this study (amoxicillin-clavulanate and enrofloxacin in 43% of patients) was similar to that proposed in previous literature (MacDonald 2010). Although current expert opinion suggests 4 to 6 weeks of antibiotic therapy (Häggström 2010), some patients in this study received much longer courses. Such long courses need to be carefully considered and patients monitored closely to determine if antibiotic ther-apy is still required as poor antimicrobial stewardship increases the risk of antimicrobial resistance (Schuts et al. 2016). Current guidelines in human cases of IE also suggest 4 to 6 weeks of antibiotic therapy, and longer courses are only indicated in cases of prosthetic valve IE (Baddour et al. 2015).
The comorbidities in dogs with endocarditis noted in this study are similar to those previously described (Sykes et al. 2006a, MacDonald 2010. The most common comorbidities were a history of osteoarthritis, skin infections and periodontal disease. Although osteoarthritis is unlikely to be related to the development of IE in dogs, it can be a precursor to immune-mediated polyarthritis or septic arthritis when combined with a generalised infection and any lameness or joint effusions should be investigated (MacDonald 2010). Skin abscesses and wounds have also been shown as portals of entry in a previous study (Sykes et al. 2006a). A link between endocarditis and periodontal disease has been shown in dogs (Pereira dos Santos et al. 2019).
One study suggests that chronic inflammation of the oral cavity in the presence of bacterial flora may lead to endocarditis due to the development of high bacteraemia particularly in dogs with stage 3 periodontal disease (Glickman et al. 2009). However, other studies challenge this association (Sykes et al. 2006a, Peddle et al. 2009). Unfortunately, the stage of dental disease was not recorded in the patients in this study. The canine oral microbiome has been shown to be highly diverse and up to 38.2% of species are unculturable, thus these may also account for some of our negative MCB (Riggio et al. 2011). Although only six of our patients presented with a history of periodontal disease, up to 64.5% of dogs are affected with the disease in the general population (Robinson et al. 2016) and so it is likely that this was under-reported in the patient records. In addition, the incidence and severity of periodontal disease increases with age which correlates with the higher number of middle age to older dogs affected by endocarditis as seen in this study (Wallis et al. 2019). From our data, endocarditis seems a rare sequela of periodontal disease.
Congenital and acquired cardiac diseases were previously shown to predispose dogs to endocarditis (Romero-Fernandez & Palermo 2019). Only two dogs had acquired cardiac disease (MMVD), thus this was not considered to be a major predisposition to IE in this referral population. Six dogs had underlying congenital heart disease (SAS) which has been previously suggested to predispose dogs to IE due to creating turbulent blood flow and damage to the aortic cusps (MacDonald 2010). In addition, SAS is one of the most common congenital heart conditions in large breed dogs which may account for their predisposition to IE (Ontiveros & Stern 2021). Male dogs have also been shown to be predisposed to SAS which may also partly explain their higher prevalence (Schrope 2015). A recent study did not diagnose any congenital SAS in their IE cases thus further studies are indicated to investigate this link (Reagan et al. 2022). One dog showed a Gerbode type defect which is thought to be secondary to destruction of the interventricular septum by bacterial IE (Peddle et al. 2008).
Contrary to previous studies (Sykes et al. 2006b, Reagan et al. 2022, the development of TED and the use of antithrombotics were not shown to have a significant effect on survival. However, it appears logical that the use of antithrombotics would be beneficial in helping to decrease the size of vegetative lesions as research has shown that lesions may shelter bacteria from the immune system (Liesenborghs et al. 2020). To confirm these results, studies with larger sample size are necessary. The development of arrhythmias was not shown to have an effect on survival in this study, as previously shown (Sykes et al. 2006b). It is possible that in the majority of cases that developed arrhythmias they were not severe or prolonged enough to affect survival. Previous studies indicated that the development of AKI was associated with mortality; however, too few dogs developed AKI in this study to allow analysis (Sykes et al. 2006b;Reagan et al. 2022). In fact, in agreement with Reagan et al., CHF was the only complication of IE that was found to have a significant effect on survival in this study (Reagan et al. 2022).
The most common clinical signs were non-specific and similar to those described in the literature (Peddle & Sleeper 2007). Interestingly, a new or worsening heart murmur was only diagnosed in 47 of the 77 patients. In some cases, dogs had a preexisting heart murmur and therefore did not meet this criterion. As this is a minor criterion in the modified Duke criteria, it is essential that the lack of a new or worsening heart murmur on initial examination does not rule out endocarditis as a differential diagnosis in a septic patient. Although only 30 of our patients had serum cTnI levels measured, it was not shown to be helpful as a prognostic indicator. A recent study has shown that serum cTnI concentrations above >0.625 ng/ml are supportive of a diagnosis of IE (Kilkenny et al. 2021). This cut-off could be useful as an additional minor criterion in the Duke's modified criteria; however, it has a high specificity and a low sensitivity, therefore, it must be used within the context of the overall clinical picture.
The survival to discharge of dogs with IE in this study was found to be better than in older USA studies, 68% compared to 22 and 56% previously reported (MacDonald et al. 2004, Sykes et al. 2006b). This correlates with a more recent USA study on IE which also found a higher survival to discharge (70%) (Reagan et al. 2022). Interestingly, there was no significant difference between the survival times of dogs with mitral and aortic IE compared to previous USA studies which reported mitral valve infections to have the longest survival time and aortic valve infections to have the shortest survival time (MacDonald et al. 2004). These differences may be linked to the lack of Bartonella spp. detected in our patients as infection with this bacterium has been shown to be negatively correlated with survival and preferentially infects the aortic valve (Sykes et al. 2006a). Further studies with larger sample numbers may help validate these findings in both the UK and the USA. Interestingly, dogs with both mitral and aortic valve IE had the shortest survival times as in a previous study and likely represent advanced disease leading to degenerative structural changes in the heart and therefore a worsened prognosis (Reagan et al. 2022). Of the dogs that developed a tricuspid and/ or pulmonic valve IE, only one tricuspid valve IE case had a history of having a jugular vascular access port placed. This likely would have been the portal of entry of the infection. Tricuspid and pulmonic valves are rarely affected by IE due to the higher pressures sustained on the left-sided valves which predisposes the mitral and aortic valve to endothelial damage (Frontera & Gradon 2000). It is thought the relatively higher oxygen concentration of the left-sided circulation is also more supportive of bacterial growth (Frontera & Gradon 2000).
There are a number of limitations in this study, many common to retrospective studies relying on data retrieval. One limitation is that there were multiple different operators who carried out the echocardiographic scans, which may have led to a different interpretation of echocardiographic images (i.e. a small endocarditis lesion may have been picked up by one cardiologist but not another and vice-versa). In addition, some rhythm abnormalities may have been missed depending on how long the ECG was run for. Furthermore, although the handling and analysis of the aseptic blood cultures were largely similar between each centre, a standardised protocol was not used which could cause some variation in the results. Another limitation was the lack of blood cultures that were positive and the small number of Bartonella spp. PCR assays performed. Unfortunately in this study, there were not enough data to allow the analysis of survival between different microorganism causing IE infections. Furthermore, data on routine complete blood work (haematology, biochemistry) were not analysed as part of this study but may have provided useful information to readers. In some cases, the cause of euthanasia may have been due to clients' financial concerns which may not reflect the actual outcome of IE. Unfortunately, this was unlikely to have been written in the clinical notes and must be considered when studying the outcome of the disease.
The results of this study have shown that the bacteria causing this disease are largely similar to those in USA studies, apart from the lack of Bartonella spp. and the higher prevalence of mitral compared to aortic valve endocarditis. The number of cases in this study highlights the low frequency of IE out of the total referral population. This study has shown that the mitral valve and large breed dogs appear predisposed to IE which can be caused by a variety of bacteria. Although the prognosis for the disease remains poor, once patients survive to discharge, they can survive for prolonged periods.